1. Field of the Invention
The present invention relates to a nitride semiconductor device employing a nitride semiconductor.
2. Description of the Related Art
Nitride semiconductors have been used for light emitting devices such as light emitting diode (LED) and laser diode (LD), light receiving devices such as solar batteries and optical sensors, and electronic devices such as transistors and power devices. Especially, light emitting diodes utilizing a nitride semiconductor have been widely used in a traffic signal, a large-scale display, and a back light source etc.
A nitride semiconductor light emitting device employing a nitride semiconductor basically has a structure where a p-type semiconductor layer and an n-type semiconductor layer are stacked on a substrate and electrodes are disposed respectively to electrically connect to the p-type semiconductor layer and the n-type semiconductor layer. Also, there has been a known structure for the electrode which is electrically connected to the p-type semiconductor layer, in which an electrode made of a transparent material is disposed on the entire surface of the p-type semiconductor layer and a pad electrode made of a metal is disposed thereon. A full-surface electrode made of a transparent material on the p-type semiconductor layer is provided to diffuse the electric current injected from the pad-electrode into the semiconductor, and to transmit and extract light that is emitted from the nitride semiconductor light emitting device.
A metal thin film and a conductive oxide film such as ITO, ZnO, In2O3, and SnO2 used as such full-surface electrode on the p-type semiconductor layer are described in, for example, Japanese Unexamined Patent Application Publication No. 2001-196633.
When a conductive oxide film is used as the full-surface electrode, a pad electrode is provided to connect the full-surface electrode with an external circuit. Rh, Pt, and W are cited as a metallic material having good adhesion with the conductive oxide film, and the pad electrode has been formed by using such metals.
However, in the pad electrode disposed on the conductive oxide film as described in Japanese Unexamined Patent Application Publication No. 2001-196633, adhesion strength with the conductive oxide film is not enough for providing adequate reliability.
For this reason, in the structure disclosed in Japanese Unexamined Patent Application No. 2005-11857, an opening is formed in the ITO portion so as to connect the p-side pad electrode and the p-type nitride semiconductor directly through the opening. With this structure, adhesion with the pad electrode may be improved, however, supply of electric current to the nitride semiconductor layer is not uniform enough. In addition, when the material for the pad electrode is simply selected in view of the adhesion between the conductive oxide film and the pad electrode, there has been a problem that sufficient flow of current between them can be hardly obtained.